Police say witnesses have a myriad of reasons for not cooperating -- among them is fear of reprisal.

But in other instances, it's more complicated. Witnesses or victims may have something to hide and believe that a police probe would be harmful to them.

"They may be as deeply involved, maybe not in the shooting we're investigating, but the shooting before that," Hertel said. "There are a lot of different reasons."

In the shootings near Hope Academy, the juvenile records of the victims may offer some insight into the lack of cooperation. Two, a 17-year-old boy and a girl the same age, were students at the school.

The boy's juvenile court record shows charges that date to November 2009 and include two counts of assault and battery, refusal to obey a police officer's lawful order, trespassing, two counts of carrying a concealed weapon and reckless use of a firearm.

He was not convicted of all those counts because of plea agreements.

The last charged incident involving the teen happened in July 2011 when he allegedly had a pistol in a Dodge Avenger at the corner of Scribner NW and Leonard Street. The gun may have been fired, because a notation in the teen's court file mentioned there were allegations of attempted murder.

For a period in 2011, he was was placed in an institution as a temporary ward of the court.

The 17-year-old girl was in juvenile court in 2011 for assault and battery.

The background of the other victim, a 16-year-old boy who attended Ottawa Hills High School, is not known.

Hertel said that witness cooperation "does definitely make a difference with our effectiveness."

"If people cooperate, we're probably going to solve a case like that," he said of certain shootings.

In particularly egregious cases, especially involving deadly shootings, Grand Rapids Police have been taking court action to compel witnesses to testify about what they know.

Prosecutors have used the investigative subpoena process more often lately, Hertel said. If prosecutors can prove a person is lying, they could be charged with perjury.